Add form fields and custom fields to the agreement via Adobe Sign API

I am working towards creating an agreement using Adobe Sign's Restful API 6.0.

I am closely following documentation link provided on your site as well as trying the API calls using REST API explorer.

I wanted to find out how can I add form fields while I create an agreement and came across a post that I first have to create an agreement and then update the form fields using another API call.

https://secure.na1.echosign.com/public/docs/restapi/v6#!/agreements/updateFormFields

I can totally do that but how can I associate form fields with a participant? I saw that there is a property called "assignee" which is an optional property to pass while updating the form fields.

I have two questions:

1. Where will I get participant set id that I need to pass in the assignee property? Do I have to make another API call to get the IDS? 
2. This property value is optional. If I do not pass in this value, Adobe sign will associate the form fields with which participant?
3. Is there a way to pass in the custom fields while creating an agreement thru API? I would like these custom fields to be hidden.